The invention relates to a method for balancing ozone sensors for the onboard diagnosis of a catalytic element for breaking down ozone in a vehicle.
For reasons for environmental and personal protection, the levels of pollution which result from vehicles with internal combustion engines or from the generation of power using stationary combustion installations needs to be reduced significantly.
A novel approach aimed at reducing pollution consists in actively removing pollutants not directly from the exhaust-gas stream from an internal combustion engine or a stationary combustion installation, but rather from the ambient air. This route is particularly promising for the removal of ground-level ozone, which has a considerable effect on human health on account of its strongly oxidizing action. Ozone itself is not a gas which is emitted directly and therefore cannot be removed from the exhaust-gas stream. It is formed when nitrogen oxides are present in outside air under insolation, on account of the UV content thereof, as a result of complex photochemical reaction equilibria.
Since ozone is extremely reactive, it can be successfully broken down quantitatively by means of a catalytic converter system through which air flows. These catalytic converters are extremely stable, since there is no need for any direct action from strong oxidation catalytic converters, which are highly sensitive to poisoning, such as for example platinum. Systems which substantially cause adsorption of the ozone on a surface are sufficient to achieve this effect; the ozone then breaks down instantaneously to form oxygen.
Such catalytic converter systems have long been in use in passenger aircraft which fly close to the ozone layer. There, they are used to treat the air which is passed into the passenger compartment. Recently, such systems have also been used in motor vehicles. In this case, the radiator of the vehicle is coated with the catalyst. The air, which flows in large quantities through the radiator, has ozone quantitatively removed from it, i.e. the vehicle purifies the ambient air.
When ozone catalyst systems of this type are used, the American environmental authority CARB (California Air Resources Board) grants automobile manufacturers credits relating to the exhaust-gas limits for the LEV (Low Emmission Vehicle) exhaust legislation. However, credits are only granted if there is onboard diagnosis of the ozone catalyst system.
A system of this type is described in the publication SAE Paper 2001-01-1302 “PremAir® Catalyst System—OBD Concepts”, Ronald M. Heck, Fred M. Allen, Jeffrey B. Hoke and Xiaolin Yang; Engelhard Corporation.
DE 199 24 083 A1 describes a conductivity sensor for-the detection of ozone which, by combining the semiconductor materials gallium oxide and indium oxide (Ga2O3, In2O3), on the one hand exploits the strong ozone sensitivity of indium oxide and on the other hand exploits the stable and reproducible conductivity properties of gallium oxide.
On account of the use of a plurality of, in particular two, sensors, the properties of which are only identical in an ideal scenario, for diagnosis, it is necessary to reckon with inaccuracies in the determination of the ozone concentration on account of the sensor specification and sensor aging. To minimize this influence and therefore to allow reliable diagnosis of the ozone conversion, the sensors have to be balanced or their output signals have to the checked for plausibility.
The invention is based on the object of providing a method for balancing ozone sensors for the onboard diagnosis of a catalytic element for breaking down ozone in a vehicle.
This object is achieved as described in the features of the invention in the claims.